1. Field of the Invention
This invention relates to printing or rapidly transferring fixed data to magnetic storage media and, more particularly, to incrementally printing servo data on both sides of thin recording media.
2. Description of the Prior Art
Modern magnetic recording systems have servo information or position markers written in an interleaved fashion on the same surface on which data are recorded. To simplify the system the same head is used to read the user data and the servo information. Various special formats are used for the servo information to enable using measurements and subsequent signal processing to determine the position of the read head relative to the center of the desired data track. A head movement mechanism and servo control system keep the read head close enough to the data track center to assure reliable read back of the previously written user data.
Most early hard disk systems used a special machine, a servo track writer (STW), see U.S. Pat. No. 5,333,140 to Moraru, et al., (1994), to record the servo information on the disk surface. The STW includes a clamping system to hold the hard disk drive (HDD) in a fixed reference position and an external motor with a laser or optical encoder to accurately move a reference pin that extended into the HDD. The actuator or head positioning mechanism of the HDD is biased against the pin so the write head of the HDD can be placed at any desired radius by moving the external motor according to its encoder system. The STW also includes a clock head that is temporarily placed on a surface of the disk by means of a special aperture in the HDD case. Circuitry of the STW writes a clock signal or timing reference by applying a pattern of write current to the clock head. The clock head reads the timing reference signal as the HDD write head is moved to any desired radius. Since the timing reference is fixed relative to the disk it is possible to write servo patterns of a desired form as function of radius and angle on the disk. Related STW are used for removable media such as the ZIP floppy diskette system manufactured by Iomega Corporation.
It is necessary to turn the disk through at least one revolution to write the servo information, and another fraction of a revolution is required to move the head to the next radius. Many servo patterns use the edges of special bursts or sub elements in the determination of the position from the read back signal, so it is necessary to write the servo bursts at radial displacements of a fraction of the data track width. Therefore it is usually required to write two or more servo tracks for each data track. Since HDDs now have about one hundred thousand or more data tracks, it may require tens of minutes to write the servo pattern.
Because the enclosure of the HDD is required to have openings for the clock head and for the reference pin it is necessary to use the STW in a special clean room to avoid contamination of the head-media interface. It is expensive and difficult to maintain the complex STW in such a clean environment.
A new “printing” approach was offered in U.S. Pat. No. 3,869,711 to Bernard, et al., (1975). That method was advanced recently (Ishida, T., et al., “Printed Media Technology for an Effective and Inexpensive Servo Track Writing of HDDs”, IEEE Trans. Magn., p1875, 2001 and Sugita, R., et al., “A Novel Magnetic Contact Duplication Technique for Servo-Writing on Magnetic Disks”, IEEE Trans. Magn. p2285, 2001). In that method the desired servo pattern is replicated in a “master element” consisting of a silicon substrate about one millimeter thick with strips of highly permeable cobalt about one half micron thick embedded in the silicon. The face of the master element containing the cobalt strips is placed in contact with a D.C. erased slave disk. Then a permanent magnet producing an oppositely directed field is brought close to the back surface of the master and is rotated one revolution relative to the master-slave pair. The cobalt strips shield portions of the slave disk leaving them in the original D.C. state, but gaps in the cobalt pattern allow the field to penetrate and reverse magnetization of the adjacent portions of the slave disk. This rapid transfer of the pattern to the entire surface of the slave disk, or “printing”, is done as the last step at the end of a conventional disk manufacturing line.
Important feature sizes, typically line widths and the thickness of cobalt strips, have been steadily decreasing, but the transition density of printing currently lags that of conventional write heads by a factor of about five. Therefore it was proposed in U.S. Pat. No. 6,304,407 to Baker, et al., (2001), to use the printed pattern as a reference system for self-servowriting (SSW). Because the printing method involves several processes such as optical diffraction, diffusion in the photo resist, and shadowing during sputtering of the cobalt, it is difficult to produce square corners or small radii of curvature at the ends of the cobalt strips. Therefore phase methods are used as in U.S. Pat. No. 3,686,649 to Behr, (1972), for the position information, and the phase is measured by discrete Fourier transforms (DFT) in the manner of U.S. Pat. No. 5,784,296 to Baker, et al., (1998). In this method the ends of the strips in the printed chevron components are excluded from the sample window, and pulses are measured at the long, clean edges.
After assembly of the HDD it is removed from the clean room and placed on a self-test rack where it begins its self-servowriting directed by the embedded firmware. Well known self-test methods measure possible pattern eccentricity and any minor errors of the position information for each servo block printed on the disk. Then corrections are applied for subsequent writing of a final servo pattern. The relatively low additional cost of printing one surface of a disk has eliminated the need for an expensive STW and the clean room in which to operate it.
Proper choice of geometry including width and thickness of the cobalt elements and width of the gaps is necessary to assure magnetic switching of the slave medium next to the gaps of the master without saturating the cobalt film to produce “secondary gaps” and consequent writing of spurious pulses or noise (Saito, A, et al., “Magnetic printing technique for longitudinal thin film media with high coercivity of 6000 Oe”, J. Appl. Phys., V 91, p 8688, 2002 and Baker, “Tradeoffs for magnetic printing of servo patterns”, J. Appl. Phys., p8691, 2002).
Because the master is about one millimeter thick and the magnet gap must be somewhat larger it follows that the applied field will also have a high level at the back surface of a thin slave medium. This problem is especially significant for flexible disks such as floppies or for tape systems where the front and back recording surfaces are separated by a film only about 10 microns thick. For thin recording media the prior printing methods would “print through” and corrupt the first printed surface as regions are later printed on the second surface.
Prior art was intended for printing servo patterns on a disk, and it requires fabrication of a master element covering the entire data area of the disk surface. Such a master element is expensive to make, and it is not feasible for a tape system where the recording medium may be many meters in length. The prior art also requires a clamping system that can maintain spacing of a few nanometers between the slave medium and the pattern areas of the master element. Unfortunately the surfaces of commercial hard disks are wavy and may deviate a few microns from a true plane, so the clamping is difficult.
As is well known in the disk drive industry the number of servo wedges or position bearing segments of the disk must increase as the track density increase. Drives now have a few hundred wedges and the trend is toward higher densities. The patterns are made by various processes such as fine scale lithography, which is also used in the manufacture of semiconductors and read-write heads for disk drives. Those processes are expensive and the yield is low for the prior art where all servo blocks or wedges of a disk must be of acceptable quality in the master element.
For certain floppy disk or tape or card systems and for thin hard disks with high track densities it is cost effective to print both sides of the media. The prior art for printing would erase or corrupt the pattern of the first surface if the second surface were printed later. Therefore it would not be possible to independently print patterns directly on both sides of thin media. That problem is solved by the present invention which uses the fact that the tangential field components near the poles of an electromagnet are very small even when the magnet is switched on and is producing a strong field in and over its gap. When the magnet is switched off the field also dies over the gap, and the medium can be advanced without perturbing interspersed patterns previously printed on either surface.